Tunable microwave resonant system and electric discharge device



May 26,:19 59' 2,888,600

J. A. RiCH TUNABLE MICROWAVE RESONANT SYSTEM AND ELECTRIC DISCHARGE navxcs- Filed Feb. 28. 1955 8 /-//.'s Attorney.

2,888,600 Patented May 195 9 ice TUNABLE MICROWAVE RE0NANT SYSTEM AND ELECTRIC DHSQHARGE DEVICE Joseph A. Rich, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Application February 28, 1955, Serial No. 491,007

11 Claims. (til. 315--5.53)

My invention relates to improved microwave resonant systems and more particularly to such systems including a resonant helix coupled to a cavity resonator. My invention also contemplates an improved method and means for tuning such systems.

There has been a continued demand for electron tubes capable of performing at higher and higher frequencies and at reasonable efiiciencies. Planar electrode tubes with a disc seal terminal and envelope construction have been greatly improved with respect to the upper limit of operating frequency. Transit time elfects, however, tend to limit the ultimate performance that can be expected from this type of tube. Other types of tubes that have been developed to operate at frequencies substantially higher than the planar electrode tubes have exhibited certain undesirable characteristics. In a usual form of klystron, for example, the interaction between the electron beam and the electromagnetic field of the resonator structure takes place at the edges of a gap defined by closely spaced wall portions of the resonator through which the beam passes. The performance of such a tube is limited by the product ,B R where ,8 is the gap coefiicient and R is the shunt resistance or load of the gap. The gap coeflicient ,8 has a maximum value of unity at Zero electron transit angle across the gap and decreases progressively as the transit angle increases. At high frequencies, 'it is apparent that the transit angle is substantial even though it may be minimized by increasing the beam voltage and by decreasing the gap length. There is a practical limit to the beam voltage and shorter gap lengths reduce the magnitude of R In a practical tube, it is, therefore, apparent that the maximum value of 18 R product that may be obtained represents a compromise between values of o and R and that the gap coefficient p is substantially less than unity, often in the order of .5 or .6.

In traveling wave tubes of the type using a helix or similar slow wave structure, the beam interacts with a simple succession of alternating electromagnetic fields which exist along the length of the structure." These tubes operate with reasonable elficiency at high frequencies but are structurally somewhat diflicult and expensive to build. The use of a resonant spiral along which a standing electromagnetic wave is developed in the operation of the device offers the advantage of a strong electric field compared with the slow wave structure of the traveling wave tube. Therefore, it permits the use of a much shorter helix. It also avoids many of the limitations of the other types of tubes mentioned above.

My invention relates to improved resonant systems, particularly for use as interaction structures with an electron beam and more particularly to such a system including a resonant helix and a cavity resonator closely coupled thereto which maybe tuned to tune the entire system over a substantial frequency range. The coupling between the resonant helix and the resonant cavity is obtained by matching electric fields across a gap in the wall of a surrounding resonator. The coupling gap is less than a quarter'of an axial wave length along the resonant helix and is preferably positioned axially near the maximum value of the electric field.

In a specific embodiment, one end of the resonator is also a radio frequency short circuit at the end of the helix, and, in this embodiment, the gap is adjacent the end of the helix since the axial component of electric field is a maximum where the radial component is Zero. In another embodiment, openings are provided in the end plates of substantially the inner diameter of the helix. In this modification, the standing wave of voltage along the resonant helix has a low value for a distance of one-quarter wave length from each end of the helix and the gap in the surrounding resonator is preferably positioned between one-quarter and one-half of a wave length from the end plate.

It is an'object of my invention to provide a new and improved tunable resonant system.

It is another object of my invention to provide a new and improved resonant system including a resonant helix coupled to a cavity resonator.

It is another object of my invention to provide a new and improved electron tube employing the tunable resonant helix and resonator system for interaction with an electron beam.

Further objects and advantages of my invention will become apparent as the following description proceeds, reference being had to the accompanying drawing in which Fig. 1 is an elevational view in section of a tunable resonant system embodying my invention;

Fig. 2 is an end view of the system of Fig. l with the end partially broken away to the interior structure;

Fig. 3 is an elevational'view in section of a modified form of resonant system embodying my invention;

Fig. 4 is an elevational view showing the system of Fig. 2 incorporated in an electric discharge device, and

Fig. '5 illustrates, with particular reference to the modification 'of Fig. 1', the variationin coupling between the'helix and resonator with varying'axial length g p v,

Referring now to Figs. 1 and 2 of the drawing, I have shown a preferred embodiment of my invention as including a cylindrical cavity resonator 1t) surrounding a resonantspiral or helix 11. "Theresonator'includes an outer cylindrical wall 12, an inner cylindrical wall 13, and an end wall 14. The cylindrical'wall 13 terminates in spaced relation to the end wall 14 and while the cylindrical wall may be of uniform diameter throughout its length, it is preferable to provide it with an end portion 15 which extends inwardly toward the outer surface of the helix to define with the end wall 14 an annular axially extending gap 16. In the specific embodiment illustrated, the end portion 15 is hemispherical with an opening therein a little larger than the outer diameter of the helix 11. This shape of the inner cylinder permits a substantial spacing of the cylindrical wall from the outer surface of the helix 11 throughout a major portion of its length and at the same time establishes the coupling gap 16 in proximity to the outer surface of the helix where the electric field of the helix is relatively strong. The helix terminates at one end in contact with the plate 14 and at the other end in contact with the plate 17 which electrically closes the inner cylinder 13 with respect to high frequency electric fields. In the embodiment illustrated, the plates 14 and 17 are provided with centrally located circular openings 18 of a diameter suitable for the passage of an electron beam axially through the helix 11 for interaction therewith; These openings are electrically closed with respect to high frequency fields by means of fine wires or mesh 19 secured to the plates 14 and 17.

In accordance with my invention, the resonator system is made tunable and this is accomplished by means for changing the resonant frequency of the cavity resonator. In the particular embodiment illustrated, the annular end wall 20 is adjustable in axial position and makes contact with the cavity walls 12 and 13 by means of spring fingers 21. A conductor 22 extending through the end wall 20 and terminating in an inductive loop 23 provides means for supplying high frequency energy to or extracting it from the resonant System. it is apparent that other known means for tuning cavity resonators may be used in place of the plunger 20.

In the modification illustrated, end wall is shown as making sliding contact with the inner wall of cylinder 13 through contact fin ers 24 to permit the position of the inner cylinder to be adjusted and in this Way to adjust the axial length of the coupling gap 16. It will be appreciated that for a particular application, this adjustment may be unnecessary.

As indicated earlier in the specification, it is an object of this invention to make use of a resonant slow wave structure, such as a helix, for interaction with an electron beam and to effect tuning of this slow wave structure by coupling it with a tunable cavity resonator in such a way that tuning of the resonator tunes the system as a whole. This result is accomplished provided the coupling is adequate and the structure previously described accomplishes this end by means of the gap 16. The gap is located close to the exterior of the helix where the field is strong and it is preferably positioned to expose the field of the resonator to the field of the helix at an axial region of the helix where the axial component of the electric field is near a maximum value.

In Fig. I have shown the variation in coupling constant k with varying gap lengths measured in centimeters. Curves A and B are derived from apparatus as shown in Fig. l with a hemispherical termination of the inner wall of the resonator forming one wall of a gap. Specifically, the helix is wound with approximately 2 /2 turns per inch and is 3% inches or about 8 /2 centimeters long. The mean helix diameter is 1% inches and the helix wire diameter is inch. The ratio of the diameters of outer cavity wall, the inner cavity wall and the outer helix is in the ratio of 4:2:1. Curve A indicates the variation in k as the axial length of the gap is varied from about /2 to 3 /2 centimeters, with the helix operating in the half wave length mode, that is, when the physical length of the elix in an axial direction is equal to /2 wave length of the standing voltage wave along the helix. The coupling constant k is a measure of the maximum deviation of the system frequency from the resonant fre quency of either the cavity alone or the helix alone. in other words, the constant k is a direct indication of the effectiveness of the plunger 20 in tuning the cavity and helix resonant system as a whole. As curve A indicates, this effectiveness extends over a substantial variation in the gap length and, in terms of wave length, this variation in gap length extends from about & of a wave length to about of a wave length. Curve B shows the variation in coupling with gap length for the case where the axial length of the helix is equal to one wave length of the standing voltage wave. In this case, the length of the coupling gap is more critical and the value of k is of a lower value indicating less effectiveness of the plunger position in tuning the system as a whole. Curve C indicates the variation in coupling for the half wave length mode but with a modified form of inner conductor 13 in which this conductor is of uniform diameter throughout its length. With this construction, the coupling is about 75% of that obtained with the hemispherical termination of the inner conductor but otherwise the variation of coupling with gap length is about the same. In terms of 6L operating frequency of the system, a variation in frequency of about plus or minus 25% about a center frequency of 420 megacycles has been obtained with a variation of plunger position of 3 to 13 centimeters from the end wall 14 (Fig. 1) and with a gap spacing of l centimeter.

In Fig. 3 of the drawing, 1 have shown a modification of my invention which is in general similar to Fig. 1 and corresponding parts have been designated by the same reference numerals. Where the parts have been modified in shape or position, the numerals have been primed. As shown in Fig. 3, the openings 18' at the ends of the helix are substantially equal in diameter to the inner diameter of the helix ill and are not covered by wires or mesh as they are in Fig. 1. With this arrangement, the standing voltage wave has a very small amplitude for a distance of A wave length from each end of the helix and the helix accordingly must be operated in a higher mode than the half wave length mode. In the embodiment illustrated, the coupling gap 16 is positioned near the physical center of the helix which would correspond to a voltage maximum in the standing wave. The gap is provided by the hemispherical end portion 15 on the inner conductor 13 and the end wall 14' of the cavity resonator. The plate 54" to which one end of the helix is atttached is supported from the end wall 14' of the resonator by means of a cylinder 25 having a diameter equal to the diameter of the inner conductor 13 of the resonator.

The use of the openings 18 without any grids 19 has an advantage when using the resonant system as an interaction system with an electron beam in that problems resulting from heating of these grid wires by the beam are eliminated. While the gap may be spaced from the end of the helix as illustrated, it is also possible to obtain substantial coupling with the gap positioned adjacent the end of the helix as shown in Fig. 1. In such an arrangement, a substantial part of the coupling is between the radial field of the helix and the resonator.

In Fig. 4 is shown a resonant system of my invention incorporated in an electron tube construction as the output structure for interacting with a modulated beam. The tube structure, in general, is similar to that employed in disc seal tubes of the planar electrode type which have found wide application in circuits having medium power requirements and operating at frequencies up to 10,000 megacycles.

As illustrated, the discharge device includes a cathode and heater assembly designated by the number 27 and including a planar emitting surface 28. The cathode assembly is supported within, and in insulated relation with respect to, a grid cylinder 29 which is in turn supported from a sleeve 30 forming a part of the planar grid terminal 31. A control grid 32 is supported on the upper end of the grid cylinder in closely spaced relation with respect to the cathode 28. The side wall of the tube is completed by terminals 33 and 34 which are spaced by a ceramic cylinder insulator 35 and the cylindrical insulator 36 which spaces the terminal 31 from the terminal 33. The upper end of the envelope is closed by a cylindrical extension 37 of the terminal 34 which is joined to or formed integral with the collecting electrode 38 having a conical collection surface 38'. Terminal 33 corresponds in a general way to the end wall 14- of the resonator in Fig. l and the terminal 34 corresponds to the end wall 17 and supports within the tube envelope a portion of the resonant system including a resonant helix 39 and an inner cylindrical conductor 40 terminating in a hemispherical end portion 41, which is spaced from the terminal 33 to provide a coupling gap 42 between the field of the resonant helix and a cavity resonator which is formed in part by terminals 33 and 34 and the conductor 41). As shown, the helix is supported axially of the tube between the terminals 33 and 34- which are apertured in alignment with ends of the helix to provide a passage for the electron beam. These openings 43 are closed with respect to high frequencies by wire mesh indicated" at 44. Cylindrical conductors 45 and 46 are shown asengaging the cylindrical portion 37 of terminal 34 and terminal 33, respectively. It will be apparent that these cylinders correspond to the inner and outer cylinders of the cavity resonator as shown in Fig. 1 and that the output resonator system of the tubemay be tuned by tuning this resonator by any suitable means, such as a plunger (not shown).

In the operation of the device shown in Fig. 4, it will be apparent that an electron beam passes between the cathode surface 28 and the collector electrode 38 and that this beam may be modulated in accordance'with a signal impressed upon the control grid 32. The modulated beam in passing through the-interior of the helix of the resonant system excites the system at a frequency in accordance with the modulation. It is apparent from the foregoing description that the frequency at which the tube is to operate may be adjusted by the position of a plunger between the cavity walls 45 and 46. A tube embodying the present invention makes possible a greater gain over a greater band width than is readily accomplished with known types of planar electrode tubes of the type having the generalconstruction of the device shown in Fig. 4.

While I have shown my invention applied to an electron discharge device as an output resonant system, it is apparent that it may be used equally well for modulating the beam or for both modulating the beam and extracting energy therefrom.

In the foregoing specification, the resonant slow wave structure has been referred to as a helix. It will be understood that a double helix structure may be used if desired. Such a double helix may be either a bifilar arrangement or contrawound in which the turns of the two helices progress in opposite senses. Also slow wave structures having similar electrical properties to the helical structures mentioned may be formed by suitably slotting sheet metal tubular members.

My invention provides means for tuning a system including such a resonant slow wave structure and a resonant cavity in which the field of the slow wave structure is coupled to the resonator through an axial gap extending over only a portion of the length of the slow Wave structure.

While I have shown and described a particular embodiment of my invention, it will be apparent to those skilled in the art that modifications may be made without departing from my invention and I, therefore, aim by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A tunable resonant system comprlsing a resonator including an outer cylindrical wall, an inner cylindrical wall terminating in a hemispherical end portion having an opening therethrough, an end wall electrically closing said outer cylindrical wall and extending in spaced relation to said end portion to provide an annular gap, a resonant slow wave structure for interaction with an electron beam supported concentrically within said inner wall and extending through said opening and into contact with said end wall, and means for tuning said system.

2. A tunable resonant system comprising a resonator including an outer cylindrical wall, an inner cylindrical wall terminating in an inwardly extending end portion having an opening therethrough, an end wall electrically closing said outer cylindrical wall and extending in spaced relation to said end portion to provide an annular gap, and a spiral-shaped conductor supported concentrically within said inner wall and extending through said opening and into contact with said end wall for interaction with an electron beam, and means extending into said resonator for tuning said system.

within said inner wall and extending through said open end and into contact with said end wall for interaction with an electron beam, and means extending into said resonator for tuning said system.

4. A microwave system comprising a helix of conduct ing material resonant at a predetermined frequency for interaction with an electron beam, a cavity resonator surrounding said helix and having an opening in a wall thereof through which said helix extends, the walls of said resonator being shaped to provide a coupling gap between the standing voltage wave of said resonant helix at said frequency and the voltage wave of said resonator, said gap having a width less than A; wave length of said standing wave, and means for tuning said resonator to change the frequency of said system.

5. A microwave system comprising a helix of conducting material resonant at a predetermined frequency for interaction with an electron beam, a cavity resonator surrounding said helix and having an opening in a wall thereof through which said helix extends, the walls of said resonator being shaped to provide an annular axially extending coupling gap between the axial standing voltage wave of said resonant helix at said frequency and the voltage wave of said resonator, said gap having an axial length less than A wave length of said standing wave, and means for tuning said resonator to change the frequency of said system.

6. A resonant system comprising a resonant helix of conducting material, means for passing an electron beam through said helix including electrodes positioned at opposite ends of said helix, a resonator surrounding said helix and a conductor within said resonator surrounding said helix to shield the exterior of said helix from the interior of said resonator throughout a substantial portion of the length of said helix, said conductor cooperating with a wall of said resonator to provide a coupling gap having a length less than A wave length of the standing voltage wave of said helix and means for tuning said system by tuning said cavity resonator.

7. A resonant system comprising a pair of apertured metal members supported in spaced relation, a helix of conducting material supported between said members for interaction with an electron beam, and a cylindrical member connected With one of said members and surrounding said helix throughout a substantial part of the length thereof and cooperating with the other of said pair of members to provide a coupling gap between the exterior of said helix and the space between said members.

8. An electric discharge device comprising a pair of apertured metal members. supported in spaced relation, a helix of conducting material supported between said members, a cylindrical member connected with one of said members and surrounding said helix throughout a substantial part of the length thereof and cooperating with the other of said pair of members to provide a coupling gap between the exterior of said helix and the space between said members, means for passing a beam through said helix including electrodes at opposite ends thereof and means connected with said metal members to complete a cavity resonator coupled with said helix through said gap.

9. An electric discharge device comprising a pair of apertured metal members supported in spaced relation, a helix of conducting material supported between said members, a cylindrical member connected with one of said members and surrounding said helix throughout a substantial part of the length thereof and cooperating with the other of said pair of members to provide a coupling gap between the exterior of said helix and the space between said members, means for passing a beam through said helix including electrodes at opposite ends thereof, means connected with said metal members to complete a cavity resonator coupled with said helix through said gap, and means for tuning said resonator to tune said helix and resonator combination.

10. An electric discharge device comprising a pair of planar apertured metal members, an envelope including a hollow cylindrical insulator interposed between said members and supporting said members in mutually spaced and insulated relationship, a helix of conducting material supported between and electrically connected at opposite ends to said members, a cylindrical metal mem ber connected with one of said members and surround ing said helix throughout a substantial part of the length thereof and cooperating with the other of said pair of members to provide a coupling gap between the exterior of said helix and the space between said members, and means for passing a beam of electrons through said helix including electrodes at opposite ends thereof.

11. An electric discharge device comprising a pair of planar apertured metal members, an envelope including a hollow cylindrical insulator interposed between said members and supporting said members in mutually spaced and insulated relationship, a resonant slow-wave structure supported between and terminated electrically at opposite ends by said members, a cylindrical metal member connected with one of said members and surrounding said slow-wave structure throughout a substantial part of the length thereof and cooperating with the other of said pair of members to provide a coupling gap between the exterior of said slow-wave structure and the space between said members, said pair of members also providing externally accessible terminals for connection with a cavity resonator, and means for passing a beam of electrons in energy exchanging relation with said slowwave structure including electrodes at opposite ends thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,615,141 Hansell Oct. 21, 1952 2,641,657 Preist et al. June 9, 1953 FOREIGN PATENTS 1,043,066 France June 10, 1953 

